Eosinophilia is characteristic of a variety of disease states ranging from asthma and atopic dermatitis to parasitic infestations and the hypereosinophilic syndrome (HES). The factors influencing stem cell commitment to the eosinophil line and subsequent excessive eosinophilopoiesis in these diseases are largely unexplored. Eosinophil colony-forming cells (EO-CFC) are the predominant colony-forming cell in normal peripheral blood, yet, circulating eosinophilis constitute only 1-3% of the normal circulating leukocyte population. The significance of this discrepancy and the role of circulating EO-CFC in production of tissue eosinophilia have not been determined. Patients with HES are heterogeneous in terms of their clinical presentations, level of blood eosinophilia and their prognosis. In our proposed studies we will quantitate EO-CFC in bone marrow and peripheral blood patients with HES and test their colony-forming ability in response to exogenous and autologous sources of colony-stimulating factor (CSF). Experiments on normal CFC will be conducted to determine whether patients with HES produce excessive quantities of EO-CSF. The ability of EO-CFC from HES patients to proliferate semiautonomously will be tested by examination of their growth potential under suboptimal culture conditions and by their ability to reproduce features of HES when injected into nude mice. Next, we will produce monoclonal antibody to EO-CFC using light density cell fractions from eosinophil colonies grown in soft agar or mythylcellulose culture as the immunogen. Monoclonal anti-EO-CFC will be screened for its reactivity against mature eosinophils, polymorphonuclear cells, fibroblasts and granulocyte-macrophage CFC. The antibody's ability to select EO-CFC from a mixed population of cells will be determined utilizing fluorescence activated cell sorting and soft agar culture of labeled and unlabeled cells. Using a fluorescent antibody technique the monoclonal antibody will be used as a probe to detect the presence of EO-CFC in tissues which normally contain large numbers of eosinophils. Lastly, we will test the ability of the monoclonal antibody both alone and when conjugated to ricin chain-A to selectively kill normal and HES EO-CFC in vitro.